1. Field of the Invention
The present invention relates to a sensor control device for driving/controlling a plurality of exhaust sensors mounted around a catalyst disposed in the exhaust passage of an internal combustion engine and for outputting sensor signals according to electric signals outputted by the exhaust sensors, to an external device.
2. Description of the Related Art
In recent years, a lean burn engine or a direct injection engine having an air/fuel ratio controlled to a leaner side than a stoichiometric value has been developed with a view to improving the fuel economy or the like. The engine of this type has a tendency to have more emission of nitrogen oxides (NOx) than that of the ordinary engine, and there have been proposed systems for the counter-measures.
One proposal is a system which is provided with a NOx occluding/reducing catalyst in the exhaust passage thereby to reduce the NOx emissions. The NOx occluding/reducing catalyst is characterized to occlude the NOx when the air/fuel ratio of the exhaust gas is lean and to reduce and release the occluded NOx when the air/fuel ratio is rich. The system adopting this catalyst executes an operation to enrich the air/fuel ratio intermittently during the lean run thereby to reduce the NOx occluded by the catalyst. In another system proposed, a NOx selective reducing catalyst is disposed in the exhaust passage, and a tank separately disposed in the vehicle is filled with urea water as a reducer solvent and injected into the catalyst so as to reduce the NOx into a harmless gas such as N2 or the like.
The internal combustion engine carrying such systems is provided with a plurality of exhaust sensors around the aforementioned catalyst (i.e., the NOx occluding/reducing catalyst or the NOx selective reducing catalyst) disposed in the exhaust passage, with a view to executing the operations such as the operation to reduce the NOx, the operation to diagnose the state (i.e., deterioration or trouble) of the catalyst and the control operation to control the air/fuel ratio. Specifically, not only a NOx sensor for measuring the NOx concentration in the exhaust gas is disposed on the downstream side of the catalyst, but also at least one of exhaust sensors including an overall air/fuel ratio sensor, a temperature sensor and a pressure sensor is disposed on the upstream or downstream side of the catalyst.
Herein, the sensors disposed in the exhaust passage for detecting pieces of information on the exhaust gas (e.g., the concentration and temperature of a specific gas), such as the NOx sensor, the overall air/fuel ratio sensor, the temperature sensor and the pressure sensor will be called altogether as the “exhaust sensors”.
Here is known an internal combustion engine having the construction, in which the electric signals outputted by the individual exhaust sensors are transmitted to an electronic control unit (as will also be called the “ECU”) for controlling the various control operations such as the ignition control operation and the aforementioned air/fuel ratio control. In this construction, a sensor control device for driving/controlling the exhaust sensors and for outputting sensor signals according to the electric signals outputted by the exhaust sensors, to the ECU is mounted in the form of an external unit.
Here, the sensor control device of the related art is disposed for each exhaust sensor. In the internal combustion engine having a plurality of exhaust sensors, a plurality of sensor control devices transmit the sensor signals individually to the ECU (as referred to JP-A-3-272450 (FIG. 2)).
Of the exhaust sensors mounted around the catalyst, the aforementioned NOx sensor is generally provided with a detection element which includes a first oxygen pumping cell, an oxygen concentration measuring cell and a second oxygen pumping cell having a pair of porous electrodes on a solid electrolyte layer. However, an electric current (or a current signal) to flow through the second oxygen pumping cell in accordance with the NOx concentration is as minute as 1 [mA] or less. If the NOx sensor and the ECU are directly connected through a signal cable, therefore, there arises a problem that the detection precision of the NOx concentration is lowered by the resistance loss due to the signal cables, the electric noises, the electric wave noises and so on. In case the NOx sensor is mounted in the exhaust passage, therefore, it has been investigated (as referred to JP-A-11-304758) to prepare the sensor control device as the external unit including the NOx sensor drive circuit and to dispose the sensor control device in the vicinity of the NOx sensor thereby to transmit the sensor signal to the ECU.